One of the difficulties when generating a terahertz signal by using a difference frequency between two continuous-wave optical signals mixed on a photoconductive switch and then detecting this terahertz signal using another photoconductive switch illuminated by the same two optical signals is the creation of a fringe pattern where the detected signal periodically goes through nulls as the terahertz wavelength is swept. This fringe pattern is the result of a path length difference between the detected terahertz signal and the local-oscillator terahertz signal generated in the receiving photoconductive switch. The fringe pattern can be eliminated by making the path length difference zero, but unfortunately this may give rise to excessive noise is the system as the phase noise of the lasers becomes correlated. To avoid this excessive noise, it would be desirable to maintain the path length difference but find some other way to prevent the nulls that occur periodically during a terahertz sweep.